From Discworld MUD Wiki
Small crystals also known as rhoksial crystal are components to the wizard spell Heezlewurst's Elemental Buffer and the long white wand. Each crystal allows protection or dealing a different damage type.
The small green crystal is also a component to the wizard spell Professor Flambardie's Grim Amulet.
They come in random quantities and colours in a small string bag in several colours
You can obtain the small string bag in the following locations:
- Mit-sin-bibh-khot's world of rare reagents
- Stapleton's Student Supplies
- The Guild of Alchemists' supply shop
The fact that they are sold together means it is less convenient if you just want one type.
See also Kefka's itemdb.